[PATCH v4 01/15] docs/mm: add document for swap table

[Kairui Song <ryncsn@gmail.com>]

From: Chris Li <chrisl@kernel.org>

Swap table is the new swap cache.

Signed-off-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>
---
 Documentation/mm/index.rst      |  1 +
 Documentation/mm/swap-table.rst | 72 +++++++++++++++++++++++++++++++++
 MAINTAINERS                     |  1 +
 3 files changed, 74 insertions(+)
 create mode 100644 Documentation/mm/swap-table.rst

diff --git a/Documentation/mm/index.rst b/Documentation/mm/index.rst
index fb45acba16ac..828ad9b019b3 100644
--- a/Documentation/mm/index.rst
+++ b/Documentation/mm/index.rst
@@ -57,6 +57,7 @@ documentation, or deleted if it has served its purpose.
    page_table_check
    remap_file_pages
    split_page_table_lock
+   swap-table
    transhuge
    unevictable-lru
    vmalloced-kernel-stacks
diff --git a/Documentation/mm/swap-table.rst b/Documentation/mm/swap-table.rst
new file mode 100644
index 000000000000..acae6ceb4f7b
--- /dev/null
+++ b/Documentation/mm/swap-table.rst
@@ -0,0 +1,72 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+:Author: Chris Li <chrisl@kernel.org>, Kairui Song <kasong@tencent.com>
+
+==========
+Swap Table
+==========
+
+Swap table implements swap cache as a per-cluster swap cache value array.
+
+Swap Entry
+----------
+
+A swap entry contains the information required to serve the anonymous page
+fault.
+
+Swap entry is encoded as two parts: swap type and swap offset.
+
+The swap type indicates which swap device to use.
+The swap offset is the offset of the swap file to read the page data from.
+
+Swap Cache
+----------
+
+Swap cache is a map to look up folios using swap entry as the key. The result
+value can have three possible types depending on which stage of this swap entry
+was in.
+
+1. NULL: This swap entry is not used.
+
+2. folio: A folio has been allocated and bound to this swap entry. This is
+   the transient state of swap out or swap in. The folio data can be in
+   the folio or swap file, or both.
+
+3. shadow: The shadow contains the working set information of the swapped
+   out folio. This is the normal state for a swapped out page.
+
+Swap Table Internals
+--------------------
+
+The previous swap cache is implemented by XArray. The XArray is a tree
+structure. Each lookup will go through multiple nodes. Can we do better?
+
+Notice that most of the time when we look up the swap cache, we are either
+in a swap in or swap out path. We should already have the swap cluster,
+which contains the swap entry.
+
+If we have a per-cluster array to store swap cache value in the cluster.
+Swap cache lookup within the cluster can be a very simple array lookup.
+
+We give such a per-cluster swap cache value array a name: the swap table.
+
+Each swap cluster contains 512 entries, so a swap table stores one cluster
+worth of swap cache values, which is exactly one page. This is not
+coincidental because the cluster size is determined by the huge page size.
+The swap table is holding an array of pointers. The pointer has the same
+size as the PTE. The size of the swap table should match to the second
+last level of the page table page, exactly one page.
+
+With swap table, swap cache lookup can achieve great locality, simpler,
+and faster.
+
+Locking
+-------
+
+Swap table modification requires taking the cluster lock. If a folio
+is being added to or removed from the swap table, the folio must be
+locked prior to the cluster lock. After adding or removing is done, the
+folio shall be unlocked.
+
+Swap table lookup is protected by RCU and atomic read. If the lookup
+returns a folio, the user must lock the folio before use.
diff --git a/MAINTAINERS b/MAINTAINERS
index 68d29f0220fc..3d113bfc3c82 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -16225,6 +16225,7 @@ R:	Barry Song <baohua@kernel.org>
 R:	Chris Li <chrisl@kernel.org>
 L:	linux-mm@kvack.org
 S:	Maintained
+F:	Documentation/mm/swap-table.rst
 F:	include/linux/swap.h
 F:	include/linux/swapfile.h
 F:	include/linux/swapops.h
-- 
2.51.0